Asteroids, comets, meteors and meteorites

Not all of the dust, ice and gas swirling around in the Solar System was eventually incorporated into the planets. At present, the leftover materials of planetary formation exist in two different classes of celestial objects: asteroids and comets.

Asteroids are small bodies consisting of solid rock or metal that orbit the Sun. Some asteroids are small planetesimals that were never incorporated into planets, while others are fragments of older, larger planetesimals that collided with each other and subsequently shattered in the early days of the Solar System. At present, most asteroids are found in the asteroid belt between the orbits of Mars and Jupiter, where Jupiter’s strong gravitational pull prevents them from coalescing into larger objects. Asteroids are irregular in shape because they are too small for their own gravity to reshape them into spheres.

Comets are icy planetesimals that generally have a highly elliptical orbit. When they are closest to the Sun, comets heat up and partially evaporate to release a glowing tail of gas and dust. They have a wide range of orbital periods and originate from two distinct regions. Comets with short orbital periods (less than 200 years) originate from the Kuiper Belt, a disk-shaped region of icy fragments beyond the orbit of Neptune, while comets with longer orbital periods originate from the Oort Cloud, a spherical region of icy fragments that extends far beyond the Kuiper Belt. These distant objects become comets when they are pulled towards the inner regions of the Solar System under the influence of gravity. Comets consist of frozen volatile compounds such as water (H2O), carbon dioxide (CO2), methane (CH4) and ammonia (NH3), together with a range of organic molecules and dust.

Even at present, astronomical objects continue to collide with the Earth. Any object that enters Earth’s atmosphere from space is called a meteoroid. These objects move so fast that friction with the atmosphere causes them to heat up and start evaporating, leaving a band of bright, glowing gas in their wake. These glowing streaks are known as meteors, although they are often incorrectly referred to as ‘falling stars’. Most meteoroids completely evaporate at high altitudes in the atmosphere, but objects that are large enough to survive the heat and reach the surface of the Earth are called meteorites. Most meteorites are rocky fragments of asteroids or planets, because the icy materials of comets are often too volatile to hit Earth’s surface.

Meteorites are divided into three broad classes: iron meteorites (made of iron-nickel alloy), stony meteorites (made of silicate rock) and stony iron meteorites (made of rock embedded in a metallic matrix). Some stony meteorites, which are referred to as carbonaceous chondrites, are derived from planetesimals that never experienced differentiation into a metallic core and rocky mantle. In contrast, other stony meteorites and all iron meteorites are derived from planetesimals that differentiated early in the history of the Solar System but subsequently disintegrated during collisions with other planetesimals. Most meteorites are approximately 4.54 billion years old, but carbonaceous chondrites are up to 4.56 billion years old – older than the oldest rocks on Earth! Because they represent primordial material that has remained unchanged all this time, they are of great importance to scientists that study the dawn of the Solar System.